Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. are powered by a source of relatively low-voltage DC power. Because power is generally delivered through a wall outlet as high-voltage AC power, a device typically referred to as a power supply or power converter is required to transform the high-voltage AC power to low-voltage DC power.
Power supplies are typically constructed by mounting electrical components onto a printed circuit board (PCB). One specific type of power supply configuration is an isolated power supply which galvanically isolates the input side from the output side of the power supply. More specifically, galvanic isolation occurs when DC current is unable to flow between the input side and output side of the power supply. One of the electrical components that is typically included in an isolated power supply is a transformer. The transformer allows the transfer of energy between an input side (referred to as a primary side) of the power supply and an output side (referred to as the secondary side) of the power supply while maintaining the galvanic isolation.
In general, power supplies include electrical circuitry that may be sensitive to electrostatic discharge (ESD). During an ESD event a charge difference increases between two objects until a sudden and momentary current is released. In other words, when a charge difference between the two objects exceeds a threshold, a current will be discharged by taking the path of least resistance between the two objects to reduce the charge difference between them. A familiar example of ESD is an electric shock one feels after walking across a carpet and then touching a metal object.
Due to the sudden current created by an ESD event, circuit functionality of an electrical device may be disrupted. In addition, other effects on electrical circuitry may include: diminishing functionality of electrical capacitors, melting of bonding wires and/or other semiconductor material, occurrences of short circuits between traces, and increasing temperature of semiconductor devices. Therefore, considerations are taken to limit and/or prevent the effect of ESD on circuitry particularly sensitive to ESD.
During an ESD event in an isolated power supply (e.g., a charged person touches the output of the power supply), a charge difference builds up between the primary and secondary sides of the power supply. If the charge difference is large enough, current may begin to flow between the secondary and primary sides of the power supply. In an effort to protect sensitive circuitry on the primary side from a potential ESD event, it is common to use an ESD conducting pathway to redirect unwanted sudden or momentary currents into an AC source coupled to the input of the power supply. Thus, during an ESD event unwanted currents will exit the power supply through this ESD conducting pathway thereby reducing the risk of damage/interference with the sensitive circuitry on the primary side of the power supply.
During power supply operation, electrical components included in the power supply, such as a transformer, may pollute nearby circuitry with unwanted electrical noise. The ESD conducting pathway may occupy a location on the PCB that allows for this electrical noise to be picked up. Electrical noise, hereon referred to as noise, may include any unwanted disturbance in the power supply, such as voltages or currents. For example, during power supply operation, the ESD conducting pathway may be exposed to changing electrical and/or magnetic fields created by electrical circuitry which may generate additional currents through the ESD conducting pathway.
Therefore, the alternative pathway for redirecting ESD may introduce noise generated from the power supply circuitry that is also redirected back to the AC source along with unwanted currents from an ESD event. By injecting additional noise back into the AC source, the power supply may produce more electromagnetic interference (EMI) than permitted by regulatory standards.